1. Field of the Invention
The present invention relates to inductors for n high frequency integrated circuits.
2. Description of the Related Art
Integrated circuits, in particular integrated circuits for wireless applications, are being driven to higher levels of integration, operation at lower supply voltages, and designs implemented for minimal power dissipation by consumer desires for low cost, small size, and long battery life. Up until this time, however, existing silicon technologies were unable to provide efficient integrable inductive structures. Losses within the semi-conducting substrate and losses due to the series resistance of the inductor's conductive path (which increase with increasing frequency of operation) were found to limit the structure's Q. The result was a limitation on a designers' ability to provide matching networks, passive filtering, inductive loading, and other inductor-based techniques on silicon integrated circuits.
Planar inductors, e.g., spiral inductors, are the type most implemented within integrated circuits. An example of a layout of a conventional integrated inductive structure is shown in FIG. 1. The key parameters in the layout of a rectangular spiral inductor are the outer dimensions of the rectangle, the width of the metal traces (i.e., conductive path), the spacing between the traces, and the number of turns in the spiral. The entire length L of the inductor's conductive path is calculated by summing each sub-length, 1.sub.1, 1.sub.2, . . . 1.sub.N. Fields created during operation by current flowing through the spiral pattern tends to cause the current to flow along the inner or shorter edges, i.e., the paths of least resistance. Current flow is believed, therefore, to be a key factor in observed increased resistance (and decreased Q) with increasing frequency.
Reducing the increase of series resistance within integrated inductive structures with increasing frequency has been accomplished by increasing the cross-sectional area of the conductive path. To do so, the metalization width, or thickness, or both is increased. Increasing the width of the inductor's conductive path up to a critical point tends to improve (minimize) resistance. However, beyond the critical point, improvement in Q begins to falter with increased width. Thereafter, current begins to flow in "limited" portions of the path's cross-section at higher frequencies. In particular, higher frequency currents tend to flow along the outer cross-sectional edges of the conductor, manifesting the so called "skin effect". Improving the magnetic coupling between adjacent runners or turns has also been found to produce an improved Q relative increased frequency.